Vesta

Phobos

Sedimentary basins on Mars, they contain a lot more sediments than we thought!

The nature of the early Martian climate is one of the major unanswered questions of planetary science. To date the geologic evidence that Mars once had large amounts of surface liquid water is conclusive, but geomorphic constraints on the duration for which that water flowed are much weaker. In addition, much of the geochemical evidence points towards surface conditions that were not warm and wet for long time periods. The evidence points towards a hydrological cycle that was intermittent and not permanently active 3.8 billion years ago. However, in a recently published article myself and colleagues report that flowing water and aqueous environments formed thick, widespread sedimentary plains 3.8 billion years ago in the northern rim of the Hellas basin on Mars.

This finding changes our understanding of the early Martian climate, as we found abundant clay deposits, stretching over a large area (>500 km), for which a large amount of water is required over a prolonged time period. This favours persistent warm and wet conditions over a long duration, rather than episodic warming events (< 10,000 years). Persistent warm and wet conditions are more conducive to the development of life.

Our discovery arose from efforts to uncover the origin of ancient plains, which by analogy with lunar plains are usually interpreted as being volcanic. Identifying the origin of martian plains is challenging because it is difficult to discriminate between volcanic flows and sedimentary rocks on heavily disrupted cratered plains (image 1). We compiled imaging and topographic data from the “High Resolution Stereo Camera” (HRSC) and “Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité” (OMEGA) instruments aboard the European Space Agency Mars Express spacecraft and from the Mars Reconnaissance Orbiter “High Resolution Imaging Science Experiment” (HiRISE) and “Compact Reconnaissance Imaging Spectrometer for Mars” (CRISM) instruments aboard the NASA Mars Reconnaissance Orbiter spacecraft. These data were used to interrogate the morphology, mineralogy and age of widespread ancient plains present at the northern rim of Hellas basin, a 2000 km diameter impact basin that formed on Mars 4.0 billion years ago.

Image 2: A) Topographic map of the Hellas Basin, where blue is low elevation and red is high elevation. Black box indicates the extent of our study area. B) Map showing the extent of the sedimentary deposits (green) and volcanic deposits (purple) found in this area. White arrows indicate wide flat sedimentary area.

From these data we made a detailed geological map of this area (image 2), taking advantage of erosional windows that form natural drill holes into the plains (image 3). Visible images and topography show that plains are predominantly formed by a thick accumulation (>500 m) of layered rocks with relatively light colours. At high resolution, these layered rocks display thin individual strata (often <1 metre) locally displaying cross-bedding stratification and networks of raised ridges. Abundant smectites – a type of clay mineral that requires an intense alteration by liquid water – were detected by infrared spectrometry in these rocks as well. These geological characteristics show that these layered rocks were not formed by lava flows, but points towards a sedimentary deposition in an aqueous environment with intense fluid circulation during burial.

We interpret these sedimentary rocks as being deposited in a lacustrine and/or alluvial context around 3.8-3.7 billion years ago, followed by an intense period of erosion 3.7-3.3 billion years ago. The estimated erosion rate for this period is two orders of magnitude higher than erosion rates estimated on Mars for more recent periods, showing a prolonged period of active surface processes during early Mars.

Clay minerals can trap organic compounds, including potential biosignatures, and indicate a habitable water-rich environment. Clay-bearing sedimentary deposits of the north Hellas basin are therefore of potential interest for robotic missions and manned missions. This detailed geological work will help in the future selection of potential landing sites in this area.

There are many motivations for studying the early climate of Mars. The first is simply that it is a fundamentally interesting unsolved problem in planetary science. Another major motivation is astrobiological — if we can understand how the Martian climate evolved, we will have a better understanding of whether life could have ever flourished, and where to look for it if it did. Studying Mars also has the potential to inform us about the evolution of our own planet, because many of the processes thought to be significant to climate on early Mars (e.g. sedimentary processes, volcanism, impacts) have also been of major importance on Earth. Finally, in this era of exoplanet science, Mars also represents a test case that can inform us about the climates of small rocky planets in general.